I have managed to get online and get these pictures:
1st picture is when an adjacent knight make shield wall, only +20 +15 in the shield wall buff tooltip without shield expert, no +5 +5bonus.
2nd picture is when an adjacent brother make shield wall, it’s +30 +23 with shield expert as you can see, having +5 +5bonus from adjacent brother’s shield wall.
And I have inferred from the hit chance that the hostile mecenery also doesnot gain bonus from friendly adjacent noble army shield wall(I know when raiders get linked shield wall the hitchance is substracted by any adjacent shield wall bonus)
So…..I think I have made my greatest effort to make things clear that friendly ally’s(not brother)shield doesnot give +5 +5bonus to my man’s shield wall….I dont know if it’s a bug or working as intended